Conventionally, phosphate glasses are widely used as optical glasses for use in electronic devices and the like. Phosphate glasses have high visible transmittance and can efficiently absorb near-infrared to infrared light depending upon composition. Therefore, phosphate glasses are widely used in the electronic field, for example, as lenses for digital cameras, heat-absorbing glass, and IR/UV-absorbing glass.
For example, Patent Literature 1 describes a substantially fluorine-free phosphate glass containing P2O5, SnO, ZnO, an alkali metal oxide, and an alkaline earth metal oxide as essential components and having an annealing point in a temperature range of 300° C. to 340° C., a refractive index of approximately 1.605, and a coefficient of linear thermal expansion ranging from 145×10−7 to 170×10−7/° C.
Patent Literature 2 describes a phosphate optical glass containing P2O5, Al2O2, and K2O or Li2O as essential components and having such optical constants as a refractive index of 1.45 to 1.65 and an Abbe's number of 65 or more.